Erasable non-volatile memory device using hole trapping in high-k dielectrics

ABSTRACT

A non-volatile memory is described having memory cells with a gate dielectric. The gate dielectric is a multilayer charge trapping dielectric between a control gate and a channel region of a transistor to trap positively charged holes. The multilayer charge trapping dielectric comprises at least one layer of high-K.

FIELD OF THE INVENTION

The present invention relates to non-volatile memory devices, and more particularly to hole trapping memory devices.

BACKGROUND

Flash memory is non-volatile, which means that it stores information on a semiconductor in a way that does not need power to maintain the information in the chip. Flash memory is based on the Floating-Gate Avalanche-Injection Metal Oxide Semiconductor (FAMOS transistor), which is essentially a Complimentary Metal Oxide Semiconductor (CMOS) Field Effect Transistor (FET) with an additional conductor suspended between the gate and source/drain terminals. Current flash memory devices are made in two forms: NOR flash and NAND flash. The names refer to the type of logic used in the storage cell array. Further, flash memory stores information in an array of transistors, called “cells,” each of which traditionally stores one or more bits of information.

A flash cell is similar to a standard Metal Oxide Semi-conductor Field Effect Transistor (MOSFET) transistor, except that it has two gates instead of just one. One gate is the control gate (CG) like in other MOS transistors, but the second is a floating gate (FG) that is insulated all around by an oxide layer. The FG is between the CG and the substrate. Because the FG is isolated by its insulating oxide layer, any electrons placed on it get trapped there and thus store the information.

When electrons are trapped on the FG, they modify (partially cancel out) an electric field coming from the CG, which modifies the threshold voltage (Vt) of the cell. Thus, when the cell is “read” by placing a specific voltage on the CG, electrical current will either flow or not flow between the cell's source and drain connections, depending on the Vt of the cell. This presence or absence of current is sensed and translated into 1's and 0's, reproducing the stored data.

A different non-volatile memory, Nitrided Read Only Memory (NROM), utilizes inherent physical features of an oxide-nitride-oxide (ONO) gate dielectric and known mechanisms of program and erase operations to create two separate physical bits per cell. The NROM cell is based on localized negative charge trapping. The cell is an n-channel MOSFET device where the gate dielectric is replaced by an ONO stack. Two spatially separated narrow charge distributions are stored in the nitride layer above junction edges. The NROM cell is programmed by channel hot electron injection.

The NROM memory devices have attracted much attention due to their advantages over the traditional floating-gate flash device, including lower programming voltage, better scalability, and improved cycling endurance. An advantage of the NROM cell is the negligible vertical retention loss due to inhibition of direct tunneling. Further, in floating gate technology the electron charge is stored in a conductive layer, and any minor oxide defect or oxide trapped charge under the gate might cause leakage and loss of all the stored charge. NROM technology, however, uses a nitride insulator as a retaining material, hence only a large defect in the oxide (comparable to the cell size) could degrade retention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a memory according to one embodiment of the invention.

FIG. 2 is a cross-section of a prior art transistor.

FIG. 3 is a cross-section of a transistor of one embodiment with a buried P-N junction.

FIG. 4 is a cross-section of a transistor of one embodiment with a multi-layered dielectric.

DESCRIPTION

In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, different embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention.

The terms wafer and substrate used in the following description include any structure having an exposed surface onto which a layer is deposited according to the present invention, for example to form the integrated circuit (IC) structure. The term substrate is understood to include semiconductor wafers. The term substrate is also used to refer to semiconductor structures during processing, and may include other layers that have been fabricated thereupon. Both wafer and substrate include doped and undoped semiconductors, epitaxial semiconductor layers supported by a base semiconductor or insulator, as well as other semiconductor structures. The term conductor is understood to include semiconductors, and the term insulator is defined to include any material that is less electrically conductive than the materials referred to as conductors.

As recognized by those skilled in the art, memory devices of the type described herein are generally fabricated as an integrated circuit containing a variety of semiconductor devices. The integrated circuit is supported by a substrate. Integrated circuits are typically repeated multiple times on each substrate. The substrate is further processed to separate the integrated circuits into dice as is well known in the art.

Relative terms such as above, below, lateral and adjacent are not limited to a specific coordinate system. These terms are used to describe relative positions between components and are not intended to be limitations. As such, additional components can be positioned between components that are above, below, lateral and adjacent to each other. Further, the figures are provided to help facilitate an understanding of the detailed description, are not intended to be accurate in scale, and have been simplified.

The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

FIG. 1 is a simplified block diagram of an integrated circuit memory device 100 in accordance with an embodiment of the invention. The memory device 100 includes an array of non-volatile memory cells 102, address circuitry 104, control circuitry 110, and Input/Output (I/O) circuitry 114.

The memory device 100 can be coupled to a processor 120 or other memory controller for accessing the memory array 102. The memory device 100 coupled to a processor 120 forms part of an electronic system. Some examples of electronic systems include personal computers, peripheral devices, wireless devices, digital cameras, personal digital assistants (PDA's) and audio recorders.

The memory device 100 receives control signals across control lines 122 from the processor 120 to control access to the memory array 102 via control circuitry 110. Access to the memory array 102 is directed to one or more target memory cells in response to address signals received across address lines 124. Once the array is accessed in response to the control signals and the address signals, data is written to or read from the memory cells across data, DQ, lines 126.

It will be appreciated by those skilled in the art that additional circuitry and control signals can be provided, and that the memory device of FIG. 1 has been simplified to help focus on the invention. It will be understood that the above description of a memory device is intended to provide a general understanding of the memory and is not a complete description of all the elements and features of a typical memory device.

In embodiments of the invention, a p-channel MOSFET with a high dielectric constant, high-K, gate insulator with hole trapping in the gate insulator is provided as a memory device. Programming can be achieved by hot hole injection from a transistor channel, light generated holes accelerated in an electric field, holes injected into the device by a buried p-n junction, or holes generated at the gate insulator-substrate interface by highly energetic electrons tunneling off of the gate. Data can be read by operating the transistor in the forward direction, or if holes are injected only near the drain by operating the transistor in the reverse direction.

Different methods of programming holes in the high-K dielectric can be employed in the present invention. Many of the available programming techniques are well known in the art and briefly explained below. For purposes of simplicity, control circuitry 110 is referred to herein as encompassing program circuitry to program a multilayer charge trapping dielectric by injecting holes onto the at least one layer of high-K dielectric.

Flash memories based on p-channel MOSFETs using hole trapping in gate oxides as a memory technique and hot hole injection are known. Further, hole trapping has been described for use in fuses and anti-fuse devices. In such memories and structures, holes from a silicon substrate are generated by large negative gate voltages, hot hole injection from the channel, or by light.

FIG. 2 depicts a simplified cross-section of a prior art metal oxide semiconductor field effect transistor (MOSFET) in a substrate 200. The MOSFET includes a source region 202, a drain region 204, and a channel region 206 in the substrate 200 between the source region 202 and the drain region 204. A gate 208 is separated from the channel region 206 by a gate oxide 210. A source line 212 is coupled to the source region 202.

In a memory device, a bitline conductor 214 is coupled to the drain region 204. A wordline conductor 216 is coupled to the gate 208. In conventional operation, a drain to source voltage potential (Vds) is set up between the drain region 204 and the source region 202. A negative voltage potential is then applied to the gate 208 via the wordline 216. Once the negative voltage potential applied to the gate exceeds the characteristic voltage threshold (Vt) of the MOSFET, the channel 206 forms in the substrate 200 between the drain region 204 and the source region 202. Formation of the channel 206 permits conduction between the drain region 204 and the source region 202, and a current (Ids) can be detected at the drain region 204.

During operation of the conventional MOSFET of FIG. 2, some change in the device drain current can be programmed for MOSFETs operated in the forward direction due to holes being trapped in the gate oxide 210 near the drain region 204. This can be accomplished by hot hole injection when the transistor is operated with a drain voltage, Vds, near the gate voltage, Vgs.

Since in this case the holes are trapped near the drain region 204, however, they are not very effective in changing the characteristics of the MOSFET. They are only effective if the transistor is operated in the reverse direction during the read cycle as in reading an NROM device. As such, hot hole injection of the prior art can be used with embodiments of the present invention.

Alternatively, a sufficiently large negative gate bias voltage can be applied to cause tunnel electrons from the gate to gain enough energy to exceed the band gap energy of the gate insulator. As a result, energetic hole-electron pairs are generated in the silicon substrate and the holes have enough energy to overcome the barrier at the insulator and substrate interface.

The holes are then injected from the substrate into the gate dielectric, where they remain trapped. A large shift in the threshold voltage of the p-channel MOSFET results. The device can subsequently be reset by applying a positive gate bias voltage. It is known in the art that the positive charge generated in gate oxides by hot hole injection can be erased by avalanche electron injection.

Another prior art method to inject holes is to generate electron hole pairs by providing incident light. The holes are accelerated towards the gate insulator or oxide and trapped in the gate insulator. Trapped positive charge results in a change in the device drain current and can be used as a memory effect or memory device. This is accomplished by hot hole injection when the transistor is operated with a drain voltage near Vgs. Erasure is achieved by hot electron injection by operation with a drain voltage, Vds, much larger than the gate voltage, Vgs.

FIG. 3 depicts a semiconductor device having a bipolar (pnp) transistor-like structure, according to one embodiment of the invention, which allows uniform injection of holes. The device includes a source region 302, a drain region 304, a back gate region 306, and a channel region 308 in the substrate 300 between the source region 302 and the drain region 304. A gate 310 is separated from the channel region 308 by a multi layer gate dielectric 312. The gate dielectric contains at least one layer of a high-K dielectric, as explained below. A source line 314 is coupled to the source region 302. A bitline conductor 316 is coupled to the drain region 304. A wordline conductor 318 is coupled to the gate 310. A terminal 320 is coupled to the back gate region 306. The back gate 306 forms a p-n junction with substrate 300.

When a positive voltage Veb is applied to the back gate region 306 via the terminal 320 and a negative voltage is applied to the gate 310 via the wordline 318, holes are injected from the p-n junction in the back gate region into the gate insulator 312. This effect is depicted in FIG. 3 and results in a change in the device threshold voltage.

Regardless of the programming method employed, embodiments of the present invention use a high-K (high dielectric constant) dielectric in the gate dielectric to trap positive charged holes. For the present embodiments, high-K dielectrics are defined as those with a dielectric constant greater than that of silicon nitride (i.e., >k=7).

FIG. 4 depicts a simplified cross-section of a metal oxide semiconductor field effect transistor (MOSFET) memory cell of the present invention. The memory cell is formed in a substrate 400. The cell includes a source region 402, a drain region 404, and a channel region 406 in the substrate 400 between the source region 402 and the drain region 404. A gate 408 is separated from the channel region 406 by a multi layer gate dielectric 410. The dielectric layers include one or more layers of high-K dielectric material.

A source line 412 is coupled to the source region 402. In a memory device, a bitline conductor 414 is coupled to the drain region 404. A wordline conductor 416 is coupled to the gate 408.

High-K dielectrics have smaller bandgap energies, and less voltage is required to inject holes into the gate insulator 410. These high-K dielectrics can be composite layers, or nanolaminates, formed by oxidation, chemical vapor deposition (CVD), evaporation, or atomic layer deposition (ALD), depending on the material used. The band gap energy of high-K dielectrics becomes smaller as the dielectric constant increases.

Example high-K dielectrics of the present invention gate dielectric include a high-K dielectric between two layers of an oxide. The high-K dielectric layer in the composite gate insulator can be selected from Table 1 and the associated fabrication techniques: TABLE 1 Dielectric ALD CVD Evaporation HfO₂ X X X ZrO₂ X X ZrSnTiO X ZrON X X ZrAlO X ZrTiO₄ X Al₂O₃ X La₂O₃ X LaAlO₃ X X HfAlO₃ X HfSiON X Y₂O₃ X Gd₂O₃ X Ta₂O₅ X TiO₂ X X Pr₂O₃ X X CrTiO₃ X YSiO X

Further examples of the present invention gate dielectric include an oxide-nitride-high-K dielectric composite layered gate insulator. The high-K dielectric layer in the composite gate insulator can be selected from ALD formed Al₂O₃, HfO₂ or ZrO₂.

Further examples of the present invention gate dielectric include three stacked layers of high-K dielectrics. The high-K dielectric layers in the composite gate insulator can be selected from dielectrics of Table 2 formed by ALD. TABLE 2 First High-K Second High-K Third High-K HfO₂ Ta₂O₅ HfO₂ La₂O₃ HfO₂ La₂O₃ HfO₂ ZrO₂ HfO₂ Lanthanide (Pr, Ne, Sm, ZrO₂ Lanthanide (Pr, Ne, Sm, Gd and Dy) Oxide Gd and Dy) Oxide Lanthanide (Pr, Ne, Sm, HfO₂ Lanthanide (Pr, Ne, Sm, Gd and Dy) Oxide Gd and Dy) Oxide

A further example of the present invention gate dielectric includes a high-K-high-K-high-K dielectric composite layered gate insulator formed comprising evaporated HfO₂ between two layers of ALD formed Lanthanide (Pr, Ne, Sm, Gd and Dy) Oxide. 

1. A non-volatile memory comprising: source and drain regions located in a transistor body region, the source and drain regions are laterally spaced apart to form a channel region therebetween; a control gate isolated from and located vertically above the channel region; a multilayer charge trapping dielectric between the control gate and the channel region to trap positively charged holes, wherein the multilayer charge trapping dielectric comprises at least one layer of high-K dielectric having a dielectric constant (K) greater than seven; and program circuitry to program the multilayer charge trapping dielectric by injecting holes onto the at least one layer of high-K dielectric.
 2. The non-volatile memory of claim 1 wherein the multilayer charge trapping dielectric comprises a layer of high-K dielectric located between first and second layers of oxide.
 3. The non-volatile memory of claim 2 wherein the layer of high-K dielectric is selected from the group HfO₂, ZrO₂, ZrSnTiO, ZrON, ZrAlO, ZrTiO₄, Al₂O₃, La₂O3, LaAlO₃, HfAlO₃, HfSiON, Y₂O₃, Gd₂O₃, Ta₂O₅, TiO₂, Pr₂O₃, CrTiO₃ and YSiO.
 4. The non-volatile memory of claim 1 wherein the multilayer charge trapping dielectric comprises an oxide layer, a nitride layer and a layer of high-K dielectric.
 5. The non-volatile memory of claim 4 wherein the layer of high-K dielectric is selected from Al₂O₃, HfO₂ or ZrO₂.
 6. The non-volatile memory of claim 5 wherein the layer of high-K dielectric is formed using an atomic layer deposition process.
 7. The non-volatile memory of claim 1 wherein the multilayer charge trapping dielectric comprises first, second and third layers of high-K dielectric.
 8. The non-volatile memory of claim 7 wherein the multilayer charge trapping dielectric comprises a layer of Ta₂O₅ located between first and second layers of HfO₂.
 9. The non-volatile memory of claim 7 wherein the multilayer charge trapping dielectric comprises a layer of HfO₂ located between first and second layers of La₂O₃.
 10. The non-volatile memory of claim 7 wherein the multilayer charge trapping dielectric comprises a layer of ZrO₂ located between first and second layers of HfO₂.
 11. The non-volatile memory of claim 7 wherein the multilayer charge trapping dielectric comprises a layer of ZrO₂ located between first and second layers of Lanthanide Oxide.
 12. The non-volatile memory of claim 7 wherein the multilayer charge trapping dielectric comprises a layer of HfO₂ located between first and second layers of Lanthanide Oxide.
 13. The non-volatile memory of claim 1 further comprising a discrete bi-polar junction located below the channel region.
 14. A memory device comprising: an array of positive charge hole trapping transistor memory cells, each memory cell comprises, source and drain regions located in a transistor body region, the source and drain regions are laterally spaced apart to form a channel region therebetween, a control gate isolated from and located vertically above the channel region, and a multilayer charge trapping dielectric between the control gate and the channel region to trap positively charged holes, wherein the multilayer charge trapping dielectric comprises at least one layer of high-K dielectric having a dielectric constant (K) greater than seven; and write circuitry to write data to the memory cells during a write operation.
 15. The memory device of claim 14 wherein the multilayer charge trapping dielectric comprises a layer of high-K dielectric located between first and second layers of oxide.
 16. The memory device of claim 14 wherein the multilayer charge trapping dielectric comprises an oxide layer, a nitride layer and a layer of high-K dielectric.
 17. The memory device of claim 14 wherein the multilayer charge trapping dielectric comprises first, second and third layers of high-K dielectric.
 18. A non-volatile transistor comprising: source and drain regions located in a transistor body region, the source and drain regions are laterally spaced apart to form a channel region therebetween; a control gate isolated from and located vertically above the channel region; and a multilayer charge trapping dielectric between the control gate and the channel region to trap positively charged holes, wherein the multilayer charge trapping dielectric comprises a layer of high-K dielectric located between first and second layers of oxide, the layer of high-K dielectric selected from the group HfO₂, ZrO₂, ZrSnTiO, ZrON, ZrAlO, ZrTiO₄, Al₂O₃, La₂O3, LaAlO₃, HfAlO₃, HfSiON, Y₂O₃, Gd₂O₃, Ta₂O₅, TiO₂, Pr₂O₃, CrTiO₃ and YSiO.
 19. A non-volatile transistor comprising: source and drain regions located in a transistor body region, the source and drain regions are laterally spaced apart to form a channel region therebetween; a control gate isolated from and located vertically above the channel region; and a multilayer charge trapping dielectric between the control gate and the channel region to trap positively charged holes, where in the multilayer charge trapping dielectric comprises an oxide layer, a nitride layer and a layer of high-K dielectric selected from Al₂O₃, HfO₂ or ZrO₂.
 20. A non-volatile transistor comprising: source and drain regions located in a transistor body region, the source and drain regions are laterally spaced apart to form a channel region therebetween; a control gate isolated from and located vertically above the channel region; and a multilayer charge trapping dielectric between the control gate and the channel region to trap positively charged holes, the charge trapping dielectric comprising first, second and third layers of high-K dielectric.
 21. The non-volatile transistor of claim 20 wherein the multilayer charge trapping dielectric comprises a layer of Ta₂O₅ located between first and second layers of HfO₂.
 22. The non-volatile transistor of claim 20 wherein the multilayer charge trapping dielectric comprises a layer of HfO₂ located between first and second layers of La₂O₃.
 23. The non-volatile transistor of claim 20 wherein the multilayer charge trapping dielectric comprises a layer of ZrO₂ located between first and second layers of HfO₂.
 24. The non-volatile transistor of claim 20 wherein the multilayer charge trapping dielectric comprises a layer of ZrO₂ located between first and second layers of Lanthanide Oxide.
 25. The non-volatile transistor of claim 20 wherein the multilayer charge trapping dielectric comprises a layer of HfO₂ located between first and second layers of Lanthanide Oxide.
 26. A method of programming a non-volatile memory transistor comprising: injecting positively charged holes into a multilayer dielectric located between a control gate and a channel region of the transistor, the multilayer dielectric comprising at least one layer of high-K dielectric having a dielectric constant (K) greater than seven; and trapping the positively charged holes in the at least one layer of high-K dielectric.
 27. The method of claim 26 wherein the injecting positively charged holes comprises hot hole injection from the channel region.
 28. The method of claim 26 wherein the injecting positively charged holes comprises light generated holes accelerated in an electric field.
 29. The method of claim 26 wherein the injecting positively charged holes comprises injecting holes via a p-n junction located below the transistor channel region.
 30. The method of claim 26 wherein the injecting positively charged holes comprises generating holes at an interface of the multi-layer dielectric and the control gate by electrons tunneling off of the control gate. 